This invention relates to electroluminescent (EL) lamps and, in particular, to large area EL lamps. As used herein, an EL “panel” is a single substrate including one or more luminous areas, wherein each luminous area is an EL “lamp”.
An EL lamp is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is transparent. Either the dielectric layer includes a phosphor powder or there is a separate layer of phosphor powder between the dielectric layer and one electrode. The phosphor powder radiates light in the presence of a strong electric field, using very little current.
A modern (post-1990) EL lamp typically includes a transparent substrate of polyester (polyethylene terephthalate, PET) or polycarbonate having a thickness of about 7.0 mils (0.178 mm). A transparent, front electrode of indium tin oxide (ITO) is vacuum deposited onto the substrate to a thickness of 1000 Å or so. A phosphor layer is screen-printed over the front electrode and a dielectric layer is screen-printed over the phosphor layer. A rear electrode is screen-printed over the dielectric layer. A rear insulation layer may be added in the form of a screen-printed layer or a tape with an adhesive coating.
The inks used for screen-printing include a binder, a solvent, and a filler, wherein the filler determines the nature of the printed layer. A typical solvent is dimethylacetamide (DMAC). The binder is typically a fluoropolymer such as polyvinylidene fluoride/hexafluoropropylene (PVDF/HFP), polyester, vinyl, or epoxy. A phosphor layer is typically screen-printed from a slurry (ink) containing a solvent, a binder, and doped zinc sulphide phosphor particles, such as described in U.S. Pat. No. 5,418,062 (Budd). A dielectric layer is typically screen-printed from a slurry (ink) containing a solvent, a binder, and barium titanate (BaTiO3) particles.
A rear (opaque) electrode is typically screen-printed from a slurry (ink) containing a solvent, a binder, and conductive particles such as silver, carbon or graphite, or mixtures thereof. A bus bar of conductive ink is typically deposited on the electrodes to reduce the voltage drop across larger lamps. When the solvent and binder for each layer are chemically the same or similar, there is chemical compatibility and good adhesion between adjoining layers. The respective layers are applied, e.g. by screen-printing or roll coating, and then cured or dried.
Most EL lamps are made in batches by screen-printing rather than being made continuously, e.g. by roll coating. As a result, the size of a lamp is limited to the size of the screen, typically eighteen by twenty-four inches (46×61 cm.). There are many applications wherein larger lamps are desired, in signage for example. What is needed is a substantially seamless, large luminous area for back lighting graphics or transparencies. “Large” is used in the sense of the EL lamp being larger than available equipment can make. Even roll coated lamps have finite width and manufacturing equipment does not scale. That is, doubling the width of a roll coated lamp is not simply a matter of doubling the width of the rolls. Uniformity of the layers becomes a significant issue as width increases, for example.
It is known in the art to provide luminous letters on a common support with the letters interconnected by cable; see for example U.S. Pat. No. 2,910,792 (Pfaff, Jr.).
In view of the foregoing, it is therefore an object of the invention to provide a substantially seamless, large EL panel.
Another object of the invention is to provide a screen printed EL lamp having an area larger than the work area of a single screen printer.
A further object of the invention is to provide a method for attaching one EL lamp to another.